Showing papers in "Soil Science in 2001"
TL;DR: In this paper, the scientific understanding of the molecular size and shape of humic substances (HS) is critically reviewed and the traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of laboratory experiments with model molecules and unwarranted assumptions.
Abstract: The scientific understanding of the molecular size and shape of humic substances (HS) is critically reviewed. The traditional view that HS are polymers in soil is not substantiated by any direct evidence but is assumed only on the basis of laboratory experiments with model molecules and unwarranted
882 citations
TL;DR: In this article, changes in concentration and quality of soil organic carbon (C) were measured in the surface 10 cm of an Oxic Paleustalf (red earth) in the semiarid area of New South Wales, Australia, at the end of 4 years under lu
Abstract: To compare the effectiveness of different pasture species in restoring soil quality, changes in concentration and quality of soil organic carbon (C) were measured in the surface 10 cm of an Oxic Paleustalf (red earth) in the semiarid area of New South Wales, Australia, at the end of 4 years under lu
507 citations
TL;DR: In this article, the potential of soil carbon to sequester carbon in the atmosphere is discussed. And the potentials of different types of soil can be reliably predicted based on the characteristics of the soil organic matter and its ability to resist microbial degradation.
Abstract: Soil carbon is a major component of the terrestrial carbon cycle. The soils of the world contain more carbon than the combined total amounts occurring in vegetation and the atmosphere. Consequently, soils are a major reservoir of carbon and an important sink. Because of the relatively long period of time that carbon spends within the soil and is thereby withheld from the atmosphere, it is often referred to as being sequestered. Increasing the capacity of soils to sequester C provides a partial, medium-term countermeasure to help ameliorate the increasing CO2 levels in the atmosphere arising from fossil fuel burning and land clearing. Such action will also help to alleviate the environmental impacts arising from increasing levels of atmospheric CO2. The C sequestration potential of any soil depends on its capacity to store resistant plant components in the medium term and to protect and accumulate the humic substances (HS) formed from the transformations or organic materials in the soil environment. The sequestration potential of a soil depends on the vegetation it supports, its mineralogical composition, the depth of the solum, soil drainage, the availability of water and air, and the temperature of the soil environment. The sequestration potential also depends on the chemical characteristics of the soil organic matter and its ability to resist microbial decomposition. When accurate information for these features is incorporated in model systems, the potentials of different soils to sequester C can be reliably predicted. It is encouraging to know that improved soil and crop management systems now allow field yields to be maintained and soil C reserves to be increased, even for soils with depleted levels of soil C. Estimates of the soil C sequestration potential are discussed. Inevitably HS are the major components of the additionally sequestered C. It will be important to know more about the compositions and associations of these substances in the soil if we are able to predict reasonably accurately the ability of any soil type to sequester C in different cropping and soil management systems.
381 citations
TL;DR: In this paper, two principles are presented that define the molecular nature and ecological role of humic substances (HS): the First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; the Second Principle makes a connection between the molecularly heterogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth.
Abstract: Two principles are presented that define the molecular nature and ecological role of humic substances (HS). The First Principle (i) accounts for and organizes an extensive body of apparently disparate data relating to the inability to purify and establish a molecular structure for HS; (ii) offers a conceptual framework for dealing with HS and for evaluating the applicability and limitations of various experimental methods; and (iii) identifies molecular heterogeneity, in combination with pronounced chemical reactivity, as constituting the essence of HS. Five corollaries to the First Principle spell out its consequences in more specific detail. New definitions of HS that offer greater insight into the molecular nature of these materials arise from the First Principle. The inapplicability of the molecular structure concept to HS is explained. The concept of hypothetical pseudostructures is introduced to help visualize the chemical reactions and interactions of HS without the unjustified assignment of specific structures to the material as a whole. Constraints in the design of experiments and in the interpretation of experimental data caused by the heterogeneous nature of HS are discussed. The Second Principle makes a connection between the molecularly heterogeneous and chemically reactive nature of HS and the ecological need for a reactive and persistent medium for plant growth. Concepts presented herein have broad implications in many fields, including chemistry, geochemistry, environmental and soil sciences, and ecology.
328 citations
TL;DR: In this paper, the major components of the mixture of materials that comprise soil organic matter, and these substances, which are by far the most abundant organic materials in the environment, are themselves complicated mixtures of biologically transformed organic debris.
Abstract: Humic substances (HS) are the major components of the mixture of materials that comprise soil organic matter, and these substances, which are by far the most abundant organic materials in the environment, are themselves complicated mixtures of biologically transformed organic debris. However, it is
233 citations
TL;DR: In this article, the origin and composition of refractory biomacromolecules and their contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.
Abstract: Extensive knowledge about refractory biomacromolecules has been accumulating for 20 years. The components, characterized by a conspicuous resistance to drastic base and acid laboratory hydrolyses, also exhibit a relatively high resistance to degradation under natural conditions. These refractory biomacromolecules, identified in vascular plants and microalgae, probably play a major role in living organism protection, and they are thought to be important for organic matter sources, composition, and turnover in soils and sediments as well. In addition, some refractory macromolecules are formed in natural environments as the result of condensation/aromatization processes. This review is concerned with six families of refractory macromolecules: lignins, sporopollenins, aliphatic macromolecules (algaenans, cutans, suberans), tannins, black carbon, and proteins. The origin and composition of each family and its contribution to organic matter in soils and sediments are discussed, focusing on recent advances and on questions that are still pending.
217 citations
TL;DR: In this article, the authors examined the effect on the release of dissolved organic carbon (DOC) of rinsing over time at two tempera- tures and under oxic and anoxic conditions in a laboratory incubation.
Abstract: Soil and plant tissues were used to examine the effect on the release of dissolved organic carbon (DOC) of rinsing over time at two tempera- tures and under oxic and anoxic conditions in a laboratory incubation. The release of DOC over 60 days of incubation ranged from 0.5 to 189 mg DOC g� 1 oven-dry material and was correlated inversely with the de- gree of decomposition of the material: fresh maple leaves > old maple leaves > Sphagnum moss > fibric peat > hemic peatsapric peat > In- ceptisol A horizon. Rates of DOC release were similar through the du- ration of the experiment, except for the fresh maple leaves, where release rates fell after 3 day. Rinsing, by the replacement of water in the incu- bating samples after 20 day, produced slower rates of DOC release, ex- cept in the Inceptisol A and sapric peat samples. There was no significant difference between DOC release under oxic and anoxic conditions, ex- cept for the Inceptisol A soil, where DOC release was greater under anoxic than under oxic conditions. The rate of DOC release at 22 � C was an average of 2.4 times greater than at 4 � C, translating into Q 10 values of about 1.6. At 22 � C under oxic conditions, DOC production accounted for 14 to 58% (average 24%) of the total C released as DOCCO 2 , with the highest proportion in the maple leaves. Under anoxic conditions, DOC production accounted for 63 to 95% (average 82%) of the total C released as DOCCO 2 � CH 4 . Production of CH 4 under anoxic con- ditions was minor, accounting for <1% of the total C released. Under oxic conditions at 22 � C, the incubations released between 2 and 107% of the organic C contained in the samples, the largest proportion of which was released from the plant tissues. Microbial utilization of DOC meant that some C was double-counted, both as DOC and as subsequently emitted CO 2 . Under anoxic conditions, 0.0 to 49% of the sample organic C was mineralized. The release of DOC represents the balance between production, adsorption, and desorption and microbial utilization. This release differs clearly among samples and among treatment effects. (Soil Science 2001;166:38-47)
178 citations
TL;DR: In this article, it was suggested that compounds with traditional structures for humic substances (HS) do not exist in soils but that soil organic matter consists instead of mixtures of plant and microbial constituents plus the same constituents in various stages of degradation.
Abstract: It is suggested that compounds with traditional structures for humic substances (HS) do not exist in soils but that soil organic matter consists instead of mixtures of plant and microbial constituents plus the same constituents in various stages of degradation. That is, HS are mixtures of plant carb
168 citations
TL;DR: In this article, the authors investigated the existence and behavior of a change point in soil P release for a wide range of variously managed soils from the United Kingdom, New Zealand, and the United States.
Abstract: Continued fertilizer applications in excess of those required for optimum plant growth can increase soil phosphorus (P) concentration and the potential for P movement to surface waters, which can contribute to freshwater eutrophication. Although soil test methods were developed for soil fertility assessment and fertilizer recommendations, they are frequently used for environmental risk assessment because of a lack of consensus on what constitutes a technically defensible environmental soil P test. Several studies have found soil test P (STP) is related to the concentration or release of P into soil solution-overland, subsurface, or drainage flow-by two linear relationships of significantly different slopes (P < 0.05) on either side of a change point for a limited number of soils. Thus, we investigated the existence and behavior of a change point in soil P release for a wide range of variously managed soils from the United Kingdom, New Zealand, and the United States. The soils varied in pH (3.0-8.2), organic C (1-172 g kg -1 ), and P (2-173 mg kg -1 as 0.5M NaHCO 3 extractable P (Olsen P) and 21-553 mg kg -1 as Mehlich-3 P). Soil P release was determined by CaCl 2 extraction (5:1 solution to soil ratio for 30 min). For all soils, CaCl 2 -P increased with STP as either Olsen or Mehlich-3 P (representing a quantity/intensity relationship typical of sorption-desorption isotherms). Statistically significant (P < 0.05) change points for Olsen P occurred in most soils (20-112 mg P kg -1 ) and for Mehlich-3 P for the United States soils (120-190 mg P kg -1 ). Soil P release (CaCl 2 -P) increased at a greater rate per unit STP,increase above these change points than below. Where no change point was detected, it was found that sampled soils were either of low or high P saturation and, thus, were grouped below or above the change point. The change point could be estimated to within 40% of the determined value with a minimum of eight randomly selected samples (4 on either side of the change point).
158 citations
TL;DR: In this paper, an air permeameter was constructed in order to measure air permeability (ka) in situ, on-site (exhumed soil samples), and in the laboratory on a wide range of Danish agricu
Abstract: The measurement of soil air permeability is a fast and easy method that can be used in different ways to characterize the soil. An air permeameter was constructed in order to measure air permeability (ka) in situ, on-site (exhumed soil samples), and in the laboratory on a wide range of Danish agricu
157 citations
TL;DR: In this paper, the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition was studied. And the effect of increasing amounts of arsenates or phosphate on the desorption or arsenate from the surfaces of Goethite was investigated.
Abstract: We studied (i) the competitive adsorption of arsenate and phosphate on goethite as affected by pH, anion concentration, and order of anion addition; (ii) the effect of increasing amounts of arsenate or phosphate on the desorption of phosphate or arsenate from the surfaces of goethite; and (iii) the
TL;DR: In this article, the authors quantified the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH.
Abstract: The sorption mechanism of glyphosate, one of the most frequently used herbicides in the world, resembles that of phosphate. This study quantifies the variation in glyphosate sorption and desorption to a coarse sandy soil and to a sandy loam soil with varying phosphorus content and pH. Using batch experiments, glyphosate adsorption and desorption isotherms were determined on soil samples taken from long-term field experiments that received different additions of phosphorus and lime during 60-year (coarse sand) and 100-year (sandy loam) periods. Sorption isotherms were non-linear and manifested adsorption desorption non-singularity. The isotherms were best fitted with an extended Freundlich model, which had earlier been shown to describe phosphate sorption data well. The phosphate content in the soils had a significant influence on the sorption of glyphosate. With 0.5 M bicarbonate extractable P (pH 8.5) increasing from 6.2 to 58.7 in the loamy sand and 9.1 to 87.4 in the coarse sand, the extended Freundlich adsorption coefficient (K f,MF,ads ) decreased from 214.7 to 106 and from 154.0 to 83.5, respectively. Liming of the coarse sandy soil resulted in stronger glyphosate sorption because of an increase of reactive amorphous aluminum and iron hydrous oxides with increasing pH values. Glyphosate competes with phosphate for sorption sites, a quality that might result in glyphosate being sorbed more weakly in soils with high phosphorus levels.
TL;DR: In this article, the authors show that land application of liquid manure can result in nutrient enrichment of subsurface drainage effluent when conditions promote leaching or macropore flow.
Abstract: Land application of liquid manure can result in nutrient enrichment of subsurface drainage effluent when conditions promote leaching or macropore flow. This contamination is most likely to occur when precipitation follows manure application closely and may cause environmental impacts to receiving wa
TL;DR: In this paper, the effects of minerals on organic matter cycling by microorganisms were investigated in dead microbial cells after 0, 15, 29, 90, 239, and 498 days of incubation of beech leaf litter mixed with quartz sand, Fe oxide, Al hydroxide, or Mn oxide.
Abstract: Minerals may protect organic matter against microbial decay, either by direct chemical and physical interactions or by inhibitory effects on the soil microbial community. To clarify the effects of minerals on organic matter cycling by microorganisms, we used amino sugars as tracers for C and N in dead microbial cells after 0, 15, 29, 90, 239, and 498 days of incubation of beech leaf litter mixed with quartz sand, Fe oxide, Al hydroxide, or Mn oxide. Beech leaf litter without addition of any mineral phases was used as the control. The results show that amino sugar concentrations increased as litter decomposition proceeded. Decreasing ratios of glucosamine to muramic acid and of glucosamine to galactosamine indicated that bacterial products accumulated increasingly relative to fungal cells with increasing incubation time. As the presence of Mn oxide promoted losses of plant-derived C, there was a more pronounced selective accumulation of the microbial-derived amino sugar C than in the other treatments. Aluminium hydroxide and Fe oxide inhibited synthesis of bacterial amino sugars by a factor of two. This resulted in lower amino sugar C proportions compared with the other treatments. Consequently, the amino sugar C proportions were sensitive to both increasing amino sugar synthesis and C mineralization rates. In contrast, the amino sugar N proportions were not affected by any mineral additions. Thus, the mere presence of minerals did not affect the cycling of N through the amino sugar pool, but minerals altered the relative proportions of N sequestered within residues of bacteria and fungi.
TL;DR: Analytical chemistry has played a pivotal role in soil science, providing an avenue for advances in knowledge and understanding of the transformation, reactivity, and occurrence of chemical compounds in soil.
Abstract: Analytical chemistry has played a pivotal role in soil science, providing an avenue for advances in knowledge and understanding of the transformation, reactivity, and occurrence of chemical compounds in soil. Organic matter, perhaps the least known area of soil, has generally suffered from a lack of
TL;DR: In this paper, a study was conducted to compare and correlate the Mehlich 3 soil extraction method with the Lancaster soil test method, and the two methods extracted calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P) using linear regression.
Abstract: A study was conducted to compare and correlate the Mehlich 3 soil extraction method with the Lancaster soil extraction method. This study compared calcium (Ca), magnesium (Mg), potassium (K), and phosphorus (P) concentrations in 422 soil samples from eight fields, which varied greatly in soil type, texture, and fertility. Each soil sample consisted of 10 subsamples collected from 0 to 15 cm deep. Soil fertility was determined using both the Mehlich 3 soil test method and the Lancaster soil test method. Ca, Mg, K, and P extracted by the two methods were compared using linear regression. The Lancaster method consistently extracted higher concentrations of the nutrients than did the Mehlich 3 method for low soil Ca concentrations. As soil Ca increased, differences between the two methods diminished until a point was reached where the methods extracted equal amounts of the nutrients. After this point, the Mehlich 3 methods extracted more of the nutrients than did the Lancaster method. This was attributed to the neutralization of the acidity in the first stage of the Lancaster method. Despite these differences, good correlations were found between the two methods for Ca, Mg, and K, with R 2 for the regression equations ranging from 0.73 to 0.80. However, P levels were not highly correlated (R 2 = 0.39) between the two methods, indicating that estimates of this nutrient determined with one method were not comparable to estimates made with the other.
TL;DR: In this paper, solid phase redistribution of heavy metals in arid-zone soils was studied for one year, and the heavy metals were fractionated into six operationally defined fractions.
Abstract: Bioavailability, toxicity, and mobility of heavy metals in soils are determined by their partitioning between solution and solid-phase and their further redistribution among solid-phase components. The wetting-drying moisture regime is one of the most important factors in controlling the physical, chemical, and biological properties of irrigated soils. Solid-phase redistribution of Cu, Cr, Ni, and Zn, added as soluble salts to two arid-zone soils incubated under a wetting-drying cycle moisture regime, was studied for 1 year. The heavy metals were fractionated into six operationally defined fractions. During the long-term process in the two soils, Cr was transferred from the carbonate fraction (CARB) into the organic matter fraction (OM), and Cu, Ni, and Zn moved from the exchangeable (EXC) and CARB fractions into the reducible oxide (RO), OM, easily reducible oxide (ERO), and residual fractions (RES) with time. The soil moisture regime strongly affects metal redistribution. Compared with a field capacity regime, soils at the wetting-drying cycle and saturated paste regimes had higher metal reactivity, resulting in the more complete movement of metals toward stable fractions, especially in the loessial soil and for Ni, Zn, and Cu.
TL;DR: In this article, a general approach was proposed to derive soil water retention characteristic from soi in order to estimate the water retention properties of soil using easily measurable soil properties, such as pH and pH.
Abstract: Soil water retention characteristic is an important property of soil. Indirect estimation of this property using easily measurable soil properties remains the research focus of many soil scientists. In this study, a general approach was proposed to derive soil water retention characteristic from soi
TL;DR: In this paper, the need for spatial information on soil properties at the field level is increasing, particularly for its applications in precision agriculture and environmental management, and one important soil property is clay content.
Abstract: The need for spatial information on soil properties at the field level is increasing, particularly for its applications in precision agriculture and environmental management. One important soil property is clay content; however, costs involved with obtaining soil data at the field scale are prohibit
TL;DR: A discussion of the basic multidimensional liquid state NMR experiments that can be applied to humic substances can be found in this article, where a range of experiments are presented.
Abstract: Multidimensional nuclear magnetic resonance (NMR) is a powerful and diverse tool for the elucidation of organic compounds and mixtures. This communication presents a discussion of the basic multidimensional liquid state NMR experiments that can be applied to humic substances. A range of experiments
TL;DR: In this article, the spatial distribution of soil C pools is characterized in the Armorican Massif (Western France) where these soils represent 20% of the organic carbon in the soil.
Abstract: Because soils are both a source and a sink for atmospheric CO2, there is an increasing need to characterize the spatial distribution of soil C pools. Large amounts of organic carbon (OC) accumulate in hydric bottom-lands soils. In the Armorican Massif (Western France) where these soils represent 20%
TL;DR: In this article, both humic matter of different origin and undisputed neutral (polysaccharides) and negatively charged (polystyrenesulphonates) polymers of known molecular weight were studied by high performance size exclusion chromatography after dissolution in mobile phases of different composition but constant ionic strength.
Abstract: The commonly accepted polymeric nature of humic substances is still a matter of debate. In this work, both humic matter of different origin and undisputed neutral (polysaccharides) and negatively charged (polystyrenesulphonates) polymers of known molecular weight were studied by high performance size exclusion chromatography after dissolution in mobile phases of different composition but constant ionic strength. Modification of the control mobile phase with methanol, hydrochloric acid, and acetic acid showed a progressive alteration of the chromatographic behavior of humic materials, but it did not alter that of polymers. Absolute size reduction of bulk humic material revealed by a refractive index detector was accompanied by a substantial decrease in absorbance of peaks indicated by the UV detector. Reduction of molecular absorptivity of humic substances with changes in solution composition was confirmed by UV spectroscopy over a wide range of wavelengths. Differences in chromatographic behavior between undisputed polymers and humic samples suggest that humic matter reflects, rather than a polymeric nature, a supramolecular association of heterogeneous molecules held together mainly by weak hydrophobic forces. The content of hydrophilic and hydrophobic carbons in humic substances appears to control the intermolecular hydrogen and dispersive bondings that ultimately stabilize humic conformations in different mobile phases. (Soil Science 2001;166:174–185)
TL;DR: In this article, the scale dependency of water permeability and air permeability was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm 3 and 6280 cm 3 ).
Abstract: Air permeability can be used to describe the structure of the soil but may also be used to predict saturated hydraulic conductivity. This raises the question of whether the two parameters exhibit the same degree of scale dependency. In this study the scale dependency of water permeability (saturated hydraulic conductivity, K w ) and air permeability (k a , at a matric water potential of -50 cm H 2 O) was tested at four different sites (three horizons at each site), by using two measurement scales (100 cm 3 and 6280 cm 3 ). No clear effect of scale on variability was observed. Air and water permeability displayed higher variabilities for two structured loamy soils compared with two sandy soils. For the more structured soils, the variability between measurements was lower for air compared with water permeability. Both air and water permeabilities were higher at the large scale compared with the small scale, but this scale-dependent difference was less pronounced in sandy soils, suggesting a smaller representative elementary volume. For three of the four soils, a highly correlated relationship between K w and k a on both small and large soil samples was observed. For the fourth soil, water retention data revealed that the samples were not sufficiently drained at -50 cm H 2 O to validate a comparison between the two parameters. Predictive K w (k a ) relations for the remaining three soils at the two scales compared favorably with a general K w (k a ) relation proposed by Loll et al. (1999). This study supports the use of a general predictive relation between k a near field capacity (at around -50 to -100 cm H 2 O) and K w , but caution should be taken if the soil has a large content of pores that will drain at or close to a matric water potential of -50 cm H 2 O.
TL;DR: In this article, a batch equilibration method was used to investigate the sorption/desorption of select model organic compounds (naphthalene, phenanthrene, and α-naphthol) by cation-saturated humic acids (HA) using a batch equilibrium method.
Abstract: Sorption/desorption of select model organic compounds (naphthalene, phenanthrene, and α-naphthol) by cation-saturated humic acids (HA) was investigated using a batch equilibration method. All sorption isotherms were nonlinear, with Freundlich exponents (N values) less than one. The N values for all sorption isotherms were nearly unchanged for Ca-HA, compared with untreated HA (H-HA), but were substantially decreased for Al-HA. For a given sorbent, N values followed the order: naphthalene > phenanthrene > α-naphthol. Hysteresis occurred in all systems except for naphthalene/H-HA and naphthalene/Ca-HA systems. The hysteresis index, evaluated by the ratio of the N values for desorption and sorption, followed the order: H-HA ≅ Ca-HA > Al-HA and naphthalene > phenanthrene > α-naphthol. The smaller the hysteresis index, the greater the degree of hysteresis. There was a good correlation (r 2 > 0.90) between hysteresis index and N values of sorption isotherms. These results can be successfully interpreted in the context of the dual-mode sorption model. Furthermore, these results suggest that type of cations can significantly influence sorption and desorption of hydrophobic organic molecules in humic substances.
TL;DR: In this article, a constant slope impedance factor (CSIF) model was proposed to predict solute diffusivities for three independent soils for which the soil-water characteristic curve (SWC) were not known.
Abstract: Solute diffusivity (ratio of diffusion coefficients in soil and free water, D s /D 0 ) is markedly soil-type dependent. Soil texture and pore size distribution govern the threshold soil-water content (θ th ) where D S /D 0 approaches zero as a result of discontinuous diffusion pathways. In a recent study (Soil Science 161:633-645), we suggested that θ th can be predicted from the soil-water characteristic curve (SWC) based on the Campbell pore size distribution parameter, b. In this study, the θ th -b expression was recalibrated based on diffusivity data for three soils (Hiroshima sand, Foulum loamy sand, and Yolo loam) measured in this study plus 20 soils reported in the literature, obtaining θ th =0.020b. As the SWC is often not measured, a second θ th expression that requires only knowledge of soil texture and bulk density was calibrated from measured data. A third expression, including both soil texture, bulk density, and Campbell b, was also calibrated and gave the most accurate description of θ th . The solute impedance factor (ratio of diffusivity by volumetric soil-water content), f 1 = D S /(θ Do), was shown to increase linearly with the water content available for diffusion, θ a =θ - θ th . The slopes of the f 1 -θ a relations were similar for most soils and did not exhibit soil-type dependency. Based on this, a so-called constant slope impedance factor (CSIF) model to predict D S (θ a )/D 0 is presented. The model can be used in combination with any of the three suggested θ th expressions. Combined with the soil-texture/bulk-density dependent θ th expression, the model accurately predicted solute diffusivities for three independent soils for which the SWC were not known.
TL;DR: In this paper, the structural changes that occur in the soil during the degradation process affecting natural grassland in the semi-arid ecosystem of the southern area of the District of the Calden, Argentina were analyzed.
Abstract: The aim of the present study is to determine quantitatively the magnitude of the structural changes that occur in the soil during the degradation process affecting natural grassland in the semi-arid ecosystem of the southern area of the District of the Calden, Argentina. Topsoil horizons show a reduction in depth from 20 cm in ungrazed sites to 16 cm in grazed sites, mainly as a consequence of soil compression caused by animal hooves. Bulk density values in the first 5 cm of the profile in grazed sites are approximately 27% higher than those in ungrazed sites. Total porosity values in the top few centimeters are, on average, 17% lower in grazed sites, primarily due to the collapse of macropores (>50 μm) and larger mesopores (50-9 μm). Water retention curves for the three depths in the different patches reflect the changes in pore space distribution. Grazing causes a sharp increase in mechanical impedance-an average increment of 210% over ungrazed values in the first 10 cm of soil. There are marked differences in the degree of aggregation in grazed sites compared with ungrazed sites. The former show a lower quantity and density of roots in the top few centimeters of the profile. Changes are also evident in average root diameter as a result of vegetation substitution brought on by overgrazing. The lack of significant differences in soil physical properties in areas subject to different grazing intensities underscores the gravity of the problem caused by animal traffic, which poses a serious threat to the conservation and recovery of the grazed patches.
TL;DR: In this article, the authors describe adsorption and desorption of atrazine in surface soils, subsoils, and sediments from different geologic settings with the Freundlich equation.
Abstract: Prediction of herbicide movement in the subsurface environment requires accurate estimates of herbicide sorption in soils and geologic materials. This information is often lacking for materials below the soil surface. We describe adsorption and desorption of atrazine in surface soils, subsoils, and sediments from different geologic settings with the Freundlich equation. Average adsorption partition coefficients (K f) for the C horizon materials were 0.43 ± 0.25 for oxidized till, 0.51 ± 0.02 for loess, and 0.55 ± 0.24 for alluvium. Adsorption of atrazine was controlled principally by organic C. Samples of unoxidized tills adsorbed atrazine at levels equivalent to surface soils. All subsurface sediments retained atrazine during desorption to a greater relative extent than that predicted from adsorption isotherms. Although sorption of atrazine in subsurface sediments is generally low, there is sufficient retention to warrant consideration of sorption processes in the assessment of atrazine transport in the subsurface environment.
TL;DR: In this article, the role of various modified clays in engineering formulations intended to minimize the application of agrochemicals, for tailoring high rate adsorbents to prevent the redistribution of both organic and inorganic contaminants in the environment, and for enhancing the degradation of toxic organics.
Abstract: In the last two decades, progress has been made in the modification of the properties of clay materials in order to produce new clay nanocomposite products that can be used for controlling soil environmental quality. Pillared organoclays and nanocomposites constitute a novel class of materials, prepared by propping apart the lamellae of 1:2 clays, thus changing their properties from hydrophilic to hydrophobic, and building an inorganic/organic hybrid by grafting interlayers with selective pore sizes or by dispersion of the mineral component as a reinforcing phase. Acid and salt treatments lead to modification of the catalytic properties of clays. Thermal and mechanical treatments are also ways to modify the capacity of clays to retain both water and chemicals. These modified clays may be used for pollution prevention and for remediation of the soil environment. Selected examples are presented to show the role of various modified clays in engineering formulations intended to minimize the application of agrochemicals, for tailoring high rate adsorbents to prevent the redistribution of both organic and inorganic contaminants in the environment, and for enhancing the degradation of toxic organics. In situ modification of the soil clay properties with the aim of increasing the retention of micropollutants and avoiding their transport into the groundwater is another important use of modified clays. This analysis of the status of studies of clay modification and use leads to the conclusion that fundamental studies at the molecular level should be the basis for future developments in producing new nanocomposite clay materials, especially multifunctional compounds able to retain/degrade organic an inorganic pollutants simultaneously.
TL;DR: In this paper, the in situ cone index, the bulk density and water content profile of a 15ha field was intensively sampled three times over a 2-year period, producing 688 data triplets These data were used to produce soil water strength-bulk density response surface relationships using robust curve fitting.
Abstract: Compaction significantly reduces yield, quality, and profitability of irrigated crops in the US Pacific Northwest (PNW) Compaction assessment is usually done via bulk density measurement, even though crops respond negatively to excessive compaction largely because of root penetration (soil strength) limitations, not because of bulk density per se For most soils, strength is thought to depend primarily on the interaction of water content and bulk density We hypothesized that the soil strength (expressed as cone index) of an important PNW soil, Portneuf silt loam (Durinodic Xeric Haplocalcid), could be predicted for a given bulk density or water content and that it would increase with increasing bulk density and decreasing water content To test this, the in situ cone index, the bulk density and water content profile of a 15-ha field was intensively sampled three times over a 2-year period, producing 688 data triplets These data were used to produce soil water strength-bulk density response surface relationships using robust curve fitting Cone index relationships were poor when derived from full-profile data sets but improved when data were segregated by depths When grouped by depth intervals, cone indices of individual layers were always correlated strongly with soil water content, but not always with bulk density The high calcium carbonate content of this soil was thought to have produced cementation effects on the cone index that varied with prolonged wetting versus prolonged drying Variability among in situ strength penetrations and bulk density cores was also thought to reduce model accuracy The difficulties inherent in developing the comprehensive relationships of soil strength to bulk density, and the overriding dependency of strength on the dynamic variable of water content, suggest great uncertainty when using bulk density sampling for realistic assessment of overall soil status affecting root restriction or crop performance unless sampling is extensive and the relationships between strength, bulk density, and water content have been intensively documented for an individual soil (Soil Science 2001;166: 520-529)